1. Field of the Invention
This invention relates to an ultraviolet crystal filter material for creating a solar blind optical filter and a UV detection system having an ultraviolet crystal filter.
2. Discussion of Related Art
Ultraviolet (UV) filters and sensors, are used for a variety of applications, including fire detection systems, electrical corona detection systems, aircraft landing aids, and missile approach warning systems. Due to the small amount of solar radiation in the UV range (i.e., wavelengths of about 350 nm and below) at low altitudes, it is possible to have UV detection in daylight conditions. UV sensors that operate in a wavelength range of between about 350 nm to about 200 nm are considered to be “Solar Blind”.
One example of a solar blind UV detection system occurs in fire alarm systems where the presence of a threshold amount of UV radiation is used to detect a fire. Upon detection of the fire, the alarm system can trigger fire alarms, fire suppression systems or calls to the local fire department.
Another example use of a solar blind UV sensor can be found in the AN/AAR-47 missile warning system. This system is typically used on helicopter platforms and utilizes UV sensors to detect infrared or radar guided missile threats. If a missile threat is detected, appropriate countermeasures, such as the ejection of flare decoys or chaff, can be deployed.
A UV optical system includes optical components for focusing radiation onto a UV sensitive sensor, a filter for reducing the amount of radiation not in the UV window (e.g., 200-350 nm), and electronics which process and act on signals from the UV sensitive sensor. To date, most UV optical filters include a series of optical filter elements that are stacked to achieve a desired spectral response. For example, a solar blind UV filter may have a pass band of between the wavelengths of about 200 nm to about 350 nm. Each optical element in the series is a single separate optical filter.
FIG. 1 shows an optical filter 1 having a single filter element, film 2, on a substrate 3. Substrate 3 can be an optically transparent layer that provides structural strength. An optically transparent layer is a layer of material transparent in the wavelength range of interest, e.g. between about 200 nm and about 350 nm for a solar blind UV filter. Alternatively, substrate 3 may be a detector or other optical element related to detection of the transmitted optical radiation. Also, film 2 may be mounted alone or stacked with other films 2 that also provides optical filtering having a particular spectral response. Typically, film 2 is of a commercially available organic dye suspended in polyvinyl alcohol plastic.
There are numerous problems with the use of organic dye films. These films degrade with exposure to fluids and temperatures above 100° C. Additionally, organic dye films typically have low transmission in the desired band-pass region. Organic dye films are also physically easily damaged. Examples of organic dyes available for films can be found in M. Kasha, Opt. Soc. Am., 38, 929, (1948), and S. F. Pellicori et al., Apl. Oct. 5, 12, 1966.
It is desirable, therefore, to provide a UV filter with better transmission characteristics within the desired band-pass region. It is also desirable for the filter to be stable at temperatures as high as 150° C. Also, it is desirable for the filters to be chemically stable, especially with respect to fluids that may be present in the working environment or during manufacturing of the filter-detector system. Additionally, it is desirable to provide filters that have low levels of fluorescence within the operating band of the filter when exposed to UV radiation.